An integrated photonic-assisted phased array transmitter for direct fiber to mm-wave links

Millimeter-wave (mm-wave) phased arrays can realize multi-Gb/s communication links but face challenges such as signal distribution and higher power consumption hindering their widespread deployment. Hybrid photonic mm-wave solutions combined with fiber-optics can address some of these bottlenecks. Here, we report an integrated photonic-assisted phased array transmitter applicable for low-power, compact radio heads in fiber to mm-wave fronthaul links. The transmitter utilizes optical heterodyning within an electronically controlled photonic network for mm-wave generation, beamforming, and steering. A photonic matrix phase adjustment architecture reduces the number of phase-shift elements from M × N to M + N lowering area and power requirements. A proof-of-concept 2 × 8 phased array transmitter is implemented that can operate from 24–29 GHz, has a steering range of 40°, and achieves 5 dBm EIRP at an optical power of 55 mW without using active mm-wave electronics. Data streams at 2.5 Gb/s are transmitted over 3.6 km of optical fiber and wirelessly transmitted attaining bit-error rates better than 10−11.

methodology regarding routing and power and phase distribution is fundamentally different, the details are presented here.
The light emanating from a laser operating at angular frequency of ω 1 (ω 2 ) is distributed equally to M (N) rows (columns). The electric field at the input of the m th row (n th column) can be expressed as e j(ω 1 t−φ m ) (e j(ω 2 t−θ n ) ) where φ m (θ n ) is the optical phase at the input of the m th row (n th column). Note that for simplicity, the amplitude of the input electric field is considered to be unity. Before the junction of a row and column, directional couplers with varying lengths tap off light from the bus waveguides which are linearly combined using a Y-junction and routed to a photodiode (PD). The coupling lengths of the directional couplers are varied so as to ensure equal amounts of power are tapped off before each junction. As such, the electric field incident on the (m, n) photodiode can be written as Accordingly, the photocurrent (which is a nonlinear function of the electric field of the light to be photodetected) at the output of the photodiode will have a DC and an mm-wave component and is given by where R is the responsivity of the photodiode and indicating the generation of an electrical signal with frequency ω 2 − ω 1 and phase θ n − φ m . Hence, the relative phase of the photocurrents between two adjacent photodiodes on row m will be ∆Θ m n,n+1 = θ n − θ n+1 and the relative phase between the photocurrents of two adjacent photodiodes on column n will be ∆Φ n m,m+1 = φ m − φ m+1 . By setting ∆Θ m n,n+1 = ∆θ and ∆Φ n m,m+1 = ∆φ , or in other words setting the relative phase shift between elements on a row (column) to be ∆θ (∆φ ), a beam that can be steered in both directions will be formed using M + N as opposed to M × N phase shifters. Note that in the presence of relative phase mismatch between adjacent elements, arising for example due to process variation across the chip, the sidelobe suppression decreases, and power in the main lobe drops. In the implemented scheme employing M + N phase shifters, this effect can be largely compensated through the optimization and calibration process described in the main text and as shown by the measurement results of Fig.   5a. Note that in phased arrays with per element amplitude and phase control, in theory, the effect of the mismatch can be fully compensated. The photonic structures such as the couplers, Y-junctions, and waveguide crosses used in the photonic distribution network were designed and simulated using Lumerical FDTD. An optical distribution network utilizing path sharing and a reduced number of phase control elements for mm-wave beam formation and steering. At each intersection point, the linear superposition of the two optical waves is photodetected resulting in an mm-wave current whose phase can be set optically.

Supplementary Note 2: Optical power distribution
A photonic network is used to distribute the laser powers equally into the array of photodiodes. Light from one laser path (red -column) utilizes 4 layers of Y-junction splitters and a Y-junction combiner while the other laser path (blue -row) uses one layer of Y-junction, cascaded directional couplers with varying lengths and a Y-junction combiner.
To demonstrate the effectiveness of the network in equal power distribution, each laser path was illuminated separately and the photocurrent after each photodiode was measured. This measurement was carried out for two different chips. The maximum photocurrent observed across both chips was then used to normalize the results in order to show the optical power mismatch between the photodiodes and demonstrate repeatability and tolerance to fabrication variation from chip to chip. Supplementary   Fig. 2 summarizes the findings of these measurements and show that the variation/mismatch generally remains under 1dB. The only exception is when light is illuminated from the blue (row) with PD1 and PD16 exhibiting less than 1.5dB variation. However, this slightly higher variation is consistent across the two chips and thus suggests that the directional coupler at the end of the bus waveguide needed a slightly longer coupling length.

Supplementary Note 3: On-PCB antenna array design
The 2×8 patch antenna array and the distribution lines were designed and simulated using Ansys HFSS 3D electromagnetic simulator. Supplementary Fig. 3a shows the structure of the patch antenna implemented on the 4-layer Rogers 4350 printed circuit board. The design is similar to a previously reported antenna 2 but scaled to 28GHz. Supplementary Fig. 3b shows the cross-section (stackup) of the PCB. The radiation pattern of a single patch antenna is shown in Supplementary Fig. 3c whereas Supplementary   Fig. 3d shows the radiation pattern of the 2×8 array including the effect of the distribution lines from the chip to the antennas (for θ = 0 • , ϕ = 0 • steering angle). The array achieves a simulated gain of around 15.1dB which takes into account the effect of the feed lines, mismatches, and losses. Microstrip transmission lines were used for the mm-wave distribution on layer L1 whereas the patch antennas were printed on L4 with L2 and L3 used as ground planes.

Supplementary Note 4: Characterization of resistive heaters
Resistive heaters based on doped silicon are used to control the phase of the optical signal in the silicon nanowaveguides using the thermo-optic effect. Such heaters are often characterized by the power required (P π ) to induce a π phase shift in the optical signal. In order to characterize the heaters and determine the P π , two heaters were placed in the arms of a Mach Zehnder Interferometer (MZI) as shown in Supplementary Fig. 4a. The output of the MZI was photodetected and the photocurrent (I PD ) was monitored. The voltage applied across the heater (V HEAT ER ) was then swept and the current flowing through the heater (I HEAT ER ) was measured. Supplementary Fig. 4b also shows a plot of I PD vs. dissipated heater power (V HEAT ER × I HEAT ER ). A power of roughly 60mW is required to induce an optical π phase shift.
It is worth mentioning that the power consumption and hence the efficiency of the thermal phase shifters can be considerably improved by using undercut cavities 3 .

Supplementary Note 7: Beam formation optimization algorithm
Supplementary Fig. 7a shows the pseudocode of the simple algorithm used for beam formation optimization. The receiving horn antenna is placed at the desired spherical location where the beam should be directed towards. The 10 on-chip 5-bit DACs controlling the heaters are sequentially swept (by increasing the current through the corresponding heaters) and the received power is simultaneously measured.
The DAC code that corresponds to the maximum received power is then saved onto that DAC before proceeding to sweep the next DAC. Supplementary Fig. 7b shows a sample optimization demonstrating how the received power changes after each step. Specifically, in this case, the horn antenna was placed perpendicular to the array at elevation and azimuth of 0 • . At the end of the optimization process, the DAC settings corresponding to the target angle are saved on a look up table. Successful beam formation is confirmed by measuring the radiation pattern.